5-Methyl-L-cysteine sulfoxide

Nishimura and coworkers studied the y-radiolysis of aqueous solutions of sulfoxide amino acids. Sulfoxide amino acids are the precursors of the flavors of onions (5-propyl-L-cysteine sulfoxide, 5-methyl-L-cysteine sulfoxide and 5-(l-propenyl)-L-cysteine sulfoxide) and garlic (5-allyl-L-cysteine sulfoxide). In studies on sprout inhibition of onion by y-irradiation it was found that the characteristic flavor of onions became milder. In the y-radiolysis of an aqueous solution of 5-propyl-L-cysteine sulfoxide (PCSO) they identified as the main products alanine, cysteic acid, dipropyl disulfide and dipropyl sulfide. In the radiolysis of 5-allyl-L-cysteine sulfoxide (ACSO) they found that the main products are S-allyl-L-cysteine, cysteic acid, cystine, allyl alcohol, propyl allyl sulfide and diallyl sulfide. The mechanisms of formation of the products were partly elucidated by the study of the radiolysis in the presence of N2O and Br as e, and OH radicals scavengers, respectively. 

Reaction 8, the direct cleavage of L-cystine to produce pyruvate and thiocysteine has been studied in much more detail. An enzyme has been found in a number of Brassica species which cattdyzes the production of pyruvate from L-cystine (Mazelis et al., 1%7). The specificity of this enzyme using six-fold purified B. napobrassica root preparations was limited to L-cystine and 5-methyl-L-cysteine sulfoxide of naturally occurring substrates. The enzyme was completely dependent on added pyridoxal 5 -phosphate. The for L-cystine was 1 vaM and 0.5 pM for pyridoxal phosphate. L-Cysteine was not a substrate but was a competitive inhibitor at low concentrations. This was due to the -SH function since glutathione had the same effect. The Aj for these compounds was 0.15 mM. 

For DMS and DMTS, 5 -methyl-L-cysteine sulfoxide is a precursor action of a C-S lyase enzyme yields methanesulfenic acid, CH3-S-OH, and hence methyl methanethiosulfinate, CH3-SO-S-CH3. Disproportionation reactions yield polysulfides such as DMS. ... 

Similar to the S-allyl-L-cysteine sulfoxide in garlic, the C-S lyase can also catalyze the flavor formation from the 5 -methyl-L-cysteine sulfoxide. Therefore, when garlic is cut or crushed, methyl sulfenic acid and allyl sulfenic acid can be produced from 5 -methyl-L-cysteine sulfoxide and S-allyl-L-cysteine sulfoxide, respectively (Figure 18.5). As sulfenic acids are quite reactive, methyl sulfenic acid and allyl sulfenic acid will quickly form the metiiyl 2-propenethiosulfinate and allyl methaneth-iosulfinate. With the similar reaction to that from diallyl thiosulfinate, these thiosulfinates produce allyl methyl disulfide and allyl methyl trisulfide. 

Contains 0.1-0.36% (usually ca. 0.2%) volatile oil, alliin (5-allyl-L-cysteine sulfoxide), 5-methyl-L-cysteine sulfoxide, enzymes (e.g., alliinase, peroxidase, and myrosinase), ajoenes ( ,Z-ajoene, ,Z-methylajoene, and dimethylajoene), protein (16.8%, dry weight basis), minerals, vitamins (thiamine, riboflavin, niacin, etc.), lipids, amino acids, and others (jiangsu karrer list and horhammer marsh martindale). ... 

Methyl L-cysteine sulfoxide 5 Chem. Ind. (London) I9S6,1428 Acta Oystallogr. 1962,15. 635... 

Methylcysteine sulfoxide accumulates in certain plants of the Cruciferae and Liliaceae (Thompson, 1967), but could not be detected in several legumes or plants of six other families examined (Fujiwaraet ai, 1958). The compound has been isolated exclusively as (-t-XS-methyl-L-cysteine sulfoxide, suggesting that it is not formed as an artifact by chemical oxidation of 5-methylcysteine. 

When onion is cut or cmshed, ( )-5 -l-propenyl-L-cysteine sulfoxide (isoalliin) is converted into ( )-l-propene-l-sulfenic acid. Different from garlic, (Z)-thiopropanal S-oxide, a lachrymatory factor, is formed from the sulfenic acid by lachrymatory factor synthase (Figure 18.6). The remaining ( )-l-propene-l-sulfenic acid and methyl sulfenic acid produced from 5 -methyl-L-eysteine sulfoxide ean form methyl 1-pro-pene-thiosulfinate and 1-propenyl methane thiosulfinate that are further converted to sulfides such as 1-propenyl methyl disulfide and 1-prope-nyl methyl trisulfide. 

The Brassica plants also obtain a flavor contribution from alkyl cysteine sulfoxide precursors. The Brassica group includes broccoli, brussel sprouts, cabbage, cauhflower, and rutabagas. S-Methyl-L-cysteine sulfoxide is the primary cysteine sulfoxide derivative found in this family. An investigation of fresh cabbage flavor found that 6% of the volatiles was dimethyl disulfide, 6% dimethyl trisulfide, 3% dimethyl tetrasulfide, and 1.5% methyl ethyl trisulfide [6]. 

Urinary metabolites are S-methylthioacetic acid sulfoxide, V-acetyl-.S -methyl-L-cysteine and /-(methylthioacetyl)glycine, which are metabolites of Y-methyl-i-cysteine and 5-methylglutathione. These last two compounds were found after incubation of methyl chloride with rodent liver, kidney and brain homogenates. The methyl group of methyl chloride is metabolized via -methyl-L-cysteine to formate which is found in urine and blood of rats, whereas formaldehyde is found in rat liver microsomes and blood of mice and rabbits (lARC, 1986). 

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